Pretensioning jacks for pretensioning reinforcing wires

ABSTRACT

A pretensioning jack has an elongate hollow body, at one end of which is provided bearing means for bearing against an anchoring plate for a reinforcing wire. Within the body is a device for gripping the projecting end of at least one reinforcing wire projecting from the anchoring plate. At the other end of the body is a traction device for drawing the gripping device away from the anchoring plate to tension the reinforcing wire or wires. Striker means are provided to force home into their seatings the wedges to anchor the wire or wires in the plate after tensioning. The gripping device includes actuating means for releasing the wire or wires after they have been tensioned.

United States Patent i 1 1111 3,762,686

Gomez Oct. 2, 1973 1 1 PRETENSIONING JACKS FOR 3,597,830 8/1971 Yegge 254 29 A PRETENSIONING REINFORCING WIRES Primary Examiner-0thell M. Simpson Att0rneyHar0ld D. Steinberg [57] ABSTRACT A pretensioning jack has an elongate hollow body, at one end of which is provided bearing means for bearing against an anchoring plate for a reinforcing wire. Within the body is a device for gripping the projecting end of at least one reinforcing wire projecting from the anchoring plate. At the other end of the body is a traction device for drawing the gripping device away from the anchoring plate to tension the reinforcing wire or wires. Striker means are provided to force home into their seatings the wedges to anchor the wire or wires in the plate after tensioning. The gripping device includes actuating means for releasing the wire or wires after they have been tensioned.

13 Claims, 3 Drawing Figures PRETENSIONING JACKS FOR PRETENSIONING REINFORCING WIRES This is a continuation, of application Ser. No. 115,434 filed Feb. l6, I971, now abandoned.

The present invention relates to pretensioning jacks for pretensioning reinforcing wires used for building reinforcement such wires or cables are conventionally disposed in bundles and anchored by means of wedges sliding in corresponding seats of an anchoring plate secured to the structure to be reinforced.

Use of the jacks employed at the present day for these purposes is complicated and consumes time and labour inasmuch as, in order to use them, it is necessary to thread the jack on the wires, secure the latter to the traction device for the wires and, after attaining the pre-determined load, to engage the anchoring wedges in the anchoring plate and release the traction device so as to be able to withdraw the jack and cut the surplus portions of the wires. the difficulties are very great if it is remembered that the forces set up are considerable and the space available therefor is reduced.

It is an object of the invention to improve this class of apparatus utilised in the building industry, in such manner as to make it possible to design them for almost complete mechanisation.

According to the invention, the jack is constituted by an elongate hollow body, preferably of tubular construction, one of the ends of which has a bearing or support head which includes a mouth for bearing against the anchoring plate associated with the reinforcing wires which the jack is required to tension. A striker means positioned inside the tubular body is axially displaceable and is associated with an actuating device for applying the striker means against the wedges of the said anchoring plate to secure the reinforcing wires after tensioning. The opposite end of the tubular body has a traction device operatively connected to a gripper-carrying head which is located also inside the tubular body. The gripper-carrying head is adapted to be coupled with the ends of the said wires and is provided with actuating means for releasing the wires after tensioning.

In the preferred embodiment of the invention, the striker means is constituted by a block or blocks sliding in an orifice coaxial with the mouth of the tubular body and formed with orifices for the passage of the wires and having furthermore an actuating face bearing against the projecting ends of the anchoring wedges, the said block being urged in the direction of the anchoring wedges by means of an annular piston which surrounds the said wires and fits slidingly in a corresponding cylinder provided at the corresponding end of the tubular body.

The traction device comprises (without excluding the possibility of using other mechanical systems) a double-action cylinder having a tubular piston coupled with a tie rod or tension member which is located in the interior of the tubular body and which is connected to the gripper-carrying head. At the upper, threaded end of the tie rod there is secured a retaining nut.

The gripper-carrying head is constituted by a block formed with orifices adapted to be engaged on the ends of the wires and provided with seats for associated sets of grippers which are movable between a gripping position and a non-gripping position and which are urged by resilient biasing means such as springs towards their gripping position, in such manner that the block is fast in rotation with the said wires. The gripper-carrying head further includes the actuating means for releasing the wires after tensioning which comprises a release plate also formed with orifices for the passage of the wires, and a sleeve mounted in each orifice surrounding the wire passing through the orifice for forcing the corresponding grippers out of their seats when the block and plate are moved axially towards each other to release the head. This relative movement may readily be achieved if the tie or tension rod of the traction device is designed to be also rotatable in its mounting and one of the blocks of the gripper-carrying head is threadingly engaged therewith, whereas the other block of the gripper-carrying head is adapted to rotate freely without any possibility for axial displacement.

Depending on application, the space between the re inforcing wires may or may not be adequate for lodging the tie or tension rod of the traction device between them. In the first case, the release plate of the grippercarrying head together with a support plate for supporting the springs urging the grippers towards their coupling position are connected for free rotation relative to the tie rod and are located axially spaced apart. The block, which carries the grippers, is connected by a screwthread to the tie rod and is axially displaceable between the release plate and the support plate.

In the second case, the gripper-carrying head is coupled to an internal screw-thread formed in a receptacle-like member which is fast with the rotary tie rod and has secured to it the support plate for the springs of the grippers, whereas the second block is mounted freely but without any possibility for axial displacement within the said receptacle-like member.

The invention is illustrated, by way of example only, in the accompanying drawings, in which FIGJ is a partial axiial section through a pretensioning jack according to the invention,

FIG.2 is an axial section through the gripper-carrying head of the jack of FIGJ, and

FIG.3 is an equivalent view of an alternative form of gripper-carrying head wherein the tie rod includes an end shaped in the form of a receptacle".

Referring to the drawings, and to FIG.1 in particular, the jack comprises a tubular body 1 the front end of which comprises a bearing or support head 2, the opposite end having a double-action hydraulic cylinder 3. The piston rod (not shown) of hydraulic cylinder 3 is tubular and carries engaged upon it a tie rod or tension member 4 provided, within the tubular body 1, with a gripper-carrying head 5 for automatic gripping of a bundle of reinforcing wires. At its other end, externally of the tubular body 1, tie rod 4 has a terminal screwthread 6 to which is coupled a retaining nut 7, for which purpose tie rod 4 is adapted to slide and rotate freely through the agency of a terminal square nut 8 or any other equivalent means.

The support or bearing head 2 is secured by means of a rear threaded seating 10 to the tubular body 1 and includes a tubular member 9. Member 9 receives, through the agency of the screws 11, a bearing or support mouth 12 having a front seating 13 for application against an anchoring plate 14 for the bundle of reinforcing wires or rods 15 disposed about the axis of the assembly. Plate 14, according to the known construction, has conical seats 16 in which the reinforcing wires or rods are retained by means of anchoring wedges or grippers 17 which can be displaced upwards as shown in FIG.1 away from their seats 16, but cannot be completely withdrawn therefrom. Rearwardly of the front seating 13, the bearing mouth 12 carries a cylindrical chuck 18 mounted in which for free sliding is an annular block 19 formed with orifices 20 for passage of the wires 15, the rear face 21 thereof projecting from the coupling face of the mouth 12 with the tubular body 9. The inner surface of the latter is defined by two chucks 22 and 23 having different diameters and between which travels a piston 24 of complimentary stepped shape so as to form between both steps a pressure chamber 25 the fluid-tightness of which is provided by two toroidal packings 26 mounted at both ends thereof in associated annular throats of the piston and of the tubular body. A plurality of helical compression springs 27, disposed between the coupling face of the bearing mouth 12 and the bottom of a plurality of bores 28 formed in the front face of the piston 24, tend to urge the piston 24 upwards as shown in FIG.1, i.e., towards the open position of the unit, however, piston 24 may be displaced in the opposite direction under the action of oil or other pressure fluid injected into the chamber 25 through conduits 29 communicating with an inlet junction 30 provided on the outer, lateral surface of the body 9 for the connection of a flexible tube extending from an appropriate feed device.

The ends of the wires 15 extending into the tube 1 are gripped by the gripper-carrying head 5 the constructional details of which are shown in FIG.2.

The end of the tie rod 4 is formed with a screwthread 31 on which is engaged for free rotation an annular gripper block 32 formed with a series of longitudinal passages or orifices 33 through which are able to enter the ends of the wires 15. The rearward facing mouths of orifices 33 are conical, thus constituting seats 34 for associated sets of grippers 35 held together by means of a resilient ring 36 lodged in an associated throat. Rearwardly of the gripper block 32, secured by means of setscrews 37, is a stop ring 38 serving as an abutment for a bearing member 39 which is free to move on the screwthread 31 and the periphery of which extends in tubular form over the head assembly, being fitted over a recess 40 in the latter, to constitute a protective means 41 for excluding dust and protecting the inner elements to be described later.

The bearing member 39 is formed with orifices 42 co-axial with those 33 in the gripper block 32 for passage of the wires 15, and internally widened or enlarged to form seating steps 43 for a plurality of springs 44 surrounding the wires 15. The free ends of springs 44 bear against the rear face ofa plate 45 formed with a central orifice 46 affording free sliding motion on the screwthread 31, and with orifices 47 which are also co-axial with those in the gripper block 32. The orifices 47 have fitted on them a plurality of flanged caps which have been given the general reference numeral 48. Caps 48 each have a flange 49 and a cylindrical shank or stem 50 and are each mounted so that the flange 49 is trapped, under the compression of the springs 44, between the free ends of the grippers 35 and the plate 45, whereas the cylindrical shank or stem 50 thereof projects from the opposite face of the plate 45 and is lodged within the springs 44, constituting centering members therefor when no wires or rods 15 are engaged through the device.

The free end of the tie rod 4 has a reduced cylindrical neck 51 and a short threaded bit 52 of still smaller diameter. The former serves as a seating for ball or roller bearings 53 carrying a loosely mounted release plate 54, and the latter carries a nut 55 securing the aforesaid elements in position. The release plate 54 is formed with orifices 56 which are co-axial with the orifices 33 formed in the annular gripper block 32, through which extend the wires or rods 15 into the device, this being facilitated by the fact that the external mouths thereof are chamfered to constitute appropriate inlets 57. The inner ends of the said orifices 56 constitute seats 58 engaged in which is a plurality of sleves 59 surrounding the wires and having thin-walled end portions 60 adapted to be introduced through the orifices 33 and to bear against the inner ends of the grippers or wedges 35. The release plate 54, like the plate 39, carries a dust-excluding means 61 which is fitted against a seat 62 on the gripper block 32, in this way closing the front portion of the device In the preferred embodiment, the sleeves 59 always extend into the corresponding orifices 33 even during tensioning. The sleeves being of a length and a diameter, however, so that when the release plate 54 and block 32 are separated a predetermined distance, as shown in FIG. 2, the sleeves do not extend into the corresponding conical mouths formed by seats 34..

The gripping device described and illustrated in F162 is used in cases wherein the distance between the wires or rods 15 leaves sufficient space for the arrangement of the tie rod 4 between them. In places wherein this is not possible, there is provided, again in accordance with the invention, the gripping device shown in F163.

Referring to FIG. 3 the front end of the tie rod 4 is formed with a screwthread 63 on which is rigidly secured a receptacle-like member or cage 64 surrounding the entire apparatus for carrying the grippers. Within member 64 there is a screwthread 65 engaged with which is a block 66 correspondingly screwthreaded so that it may be displaced therealong. The block 66 is formed with orifices and carries automatic grippers or wedges 35a equivalent to those previously described.

The rear face of the block 66 has secured to it, by means of screws 67 and spacers 68, a disc 69 fulfilling the same functions as the bearing member 39 previously described.

The bushing, gland or cap in F163 corresponding to cap 48 in FIG. 2 is shorter in this case and the intermediate plate 45 has been dispensed with.

The sleeves 59 of the device of FIG. 2 are, in this case, mounted on a disc 70 the edge of which is formed with an annular throat 71 constituting a travel path or track for a series of balls 72 which constitute an antifriction bearing with the outer ring 73. Ring 73 is formed with the inlet for the balls 74, which is closed by means of a lid or cover 75, and is fitted on a cylindrical seat 76; it is maintained in position by means of an annular nut 77 engaged in a corresponding screwthread 78 formed at the mouth of the member 64.

The mode of operation of the jack described will be clear from the drawings and from the foregoing description.

Assuming that the device is in the position shown in FIG. 1, the said device is engaged on the free ends of the wires or rods 15 projecting from the anchoring plate 14. Thereby, the wires extend through the orifices 20, through the interior of the piston 24, into the tubular body 1 and, then, through the orifices 56, 33 and 42 formed in the gripper-carrying head 5 and through the intermediate elements, as will be appreciated form the drawings.

The grippers 35 are able to open readily under the thrust of the ends of the wires 15, overcoming the resistance offered by the springs 44, but provision for the automatic opening thereof may also be made by forming them with an inner recess 79 in the rear mouth thereof and in the zone wherein are located the rings 36, so that, on the gripper block 32 being displaced downwards, leaving the gripper members 35 free, the latter automatically open their points" under the action of the rings 36.

In every case, on the device being disposed in the position shown in FIG. 2, the springs 44 introduce the grippers 35 into the interior of the seats 34 thereof which, due to their conicity, apply them against the surface of the wires, in this way initiating the automatic gripping of the head.

From this instant on, the tensioning of the reinforcing wires is carried into effect in the conventional manner, utilising the cylinder 3. The reaction of the apparatus is applied, through the agency of the bearing or support mouth 12, against the anchoring plate 14 and the wires or rods 15 are stretched or extended upwards as shown in the drawings relative to the latter.

When the predetermined load has been achieved, oil is injected into the chamber 25 of the hearing or support head 2, in such manner that the piston 24 is displaced downwards (as shown in the drawings) and urges the block 19 in the corresponding manner; the front face of the block 19, bearing against the rear ends of the anchoring wedges l7, introduces them into the seats 16 thereof, in such manner as to lock them for retaining the reinforcing wires or rods when, subsequently, the double-action cylinder 3 is slackened.

Once the device has been slackened, the tie rod 4 is rotated by means ofa spanner applied to the square nut 8, to the tie rod 4, to cause the gripper block 32, which is prevented from rotating by the wires 15, to be displaced downwards as shown in the drawings and the grippers 35 to be released. 0n subsequent displacement of the gripper block 32 in the same direction, the grippers 35 are retained at the ends of thethin-walled portions 60 and are maintained in this position in such manner that the jack may be withdrawn and is thus ready for effecting a renewed tensioning operation.

It is obvious that, if the travel path available in the double-action hydraulic cylinder 3 were inadequate for achieving the desired load in the reinforcement, the

preceding operation could be repeated before withdrawing the apparatus.

The mode of functioning in the case of the automatic gripping head shown in FIG. 3 is exactly the same as that previously described.

What is claimed is:

l. A pretensioning jack for pretensioning or prestressing reinforcing wires or rods and for anchoring the wires or rods after pretensioning or prestressing by wcdging them with wedges sliding in corresponding seats of an anchoring plate comprising:

a. an elongate hollow body for receiving the projecting portion of at least one reinforcing wire projecting from an anchoring plate for the same, said anchoring plate having i. an orifice for each said wire,

ii. at least one wedge for each said wire, and

iii. corresponding seating means for each said wire,

b. bearing means at one end of said body for bearing against said plate and having a mouth to receive said projecting portion,

0. gripper-carrying head within said body having gripper means for grippingsaid projecting portion,

d. traction means at the other end of said body for drawing said gripper-carrying head longitudinally of said projecting portion away from said mouth to tension said wire, and

e. striker means movable in said bearing means longitudinally of said projecting portion to force said wedge into engagement with its seating means to retain said wire in said plate after tensioning thereof, said gripper-carrying head including actuating means for releasing said projecting portion after tensioning of said wire.

2. A jack according to claim 1, wherein said striker means comprises a block which is slidable in said bearing means longitudinally of said projecting portion, said block having an orifice for each said wire and a bearing face for bearing against said wedge, and wherein an annular piston slidable in a cylinder in said bearing means is provided for urging said block towards said anchoring plate to urge each said wedge into engagement with its seating means to retain said wire in said anchoring plate.

3. A jack according to claim ll, wherein said traction means comprises a double-action hydraulic piston and cylinder arrangement, said piston having a tubular piston rod and a tie rod which is axially slidable in said piston rod, wherein one end of said tie rod is provided with a retaining nut and wherein said gripper-carrying head is carried at the other end of said tie rod.

4. A jack according to claim 1, wherein said grippercarrying head comprises a gripper block and a release plate, each provided with an orifice for each said wire, the gripper block and release plate being spaced from one another along said projecting portion but are movable relative to one another in the longitudinal direction of said projecting portion, said. gripper block being disposed further from said mouth than said release plate and having gripper means for each orifice and.

seating means therefor, said gripper means each being movable towards and away from said seating means between a gripping position and a non-gripping position, and resilient biasing means biasing said gripper means towards their respective gripping positions, said release plate carrying said actuating means, said actuating means comprising a sleeve lodged in each orifice of said release plate, which sleeves are disposed so that, upon movement of said gripper block and release plate towards each other, said sleeves abut against said gripper means and move them towards their non-gripping positions.

5. A jack according to claim 4, wherein said traction means comprises a piston and cylinder arrangement, said piston being coupled with a tie rod, retaining means are provided at one end of said tie rod to prevent disengagement of said tie rod from said piston, wherein said tie rod is provided at its other end with screw thread means, and wherein one of said gripper block and release plate is freely rotatable on said tie rod and the other engages with said screw thread means, whereby, upon engagement of said grippper means with said projecting portion, rotation of said tie rod in one direction about its axis causes relative movement of the gripper block and release plate away from each other to cause said gripper means to move towards their gripping positions, rotation of the other one of the gripper block or release plate being prevented by said projecting portion, and whereby rotation of said tie rod in the other direction about its axis causes relative movement of the gripper block and release plate towards each other to cause said gripper means to move towards their non-gripping positions.

6. A jack according to claim 5, wherein said release plate is freely rotatable on said tie rod, wherein a bearing member is provided on said tie rod so as to be rotatable thereabout but not axially displaceable thereon, wherein said gripper block is disposed between said bearing member and said release plate, and engages with said screw thread means, and wherein said resilient biasing means comprises springs which bear against said bearing member.

7. A jack according to claim 4, wherein said traction means comprises a piston and cylinder arrangement, said piston being coupled with a tie rod, retaining means are provided at one end of said tie rod to prevent disengagement of said tie rod from said piston, wherein said tie rod is provided at its other end with a cage member having an internal screw thread, wherein said gripper block is screw threadedly engaged with said internal screw thread, wherein said release plate is freely mounted for free rotation on said cage about an axis co-axial with the axis of said tie rod, wherein a bearing member is provided fast with said first member and wherein said resilient biasing means comprises springs which bear against said bearing member.

8. A jack according to claim 4, wherein said seating means each comprise a conical surface associated with each said orifice, wherein said gripper means comprise a plurality of wedges for each wire adapted to engage between said conical surface and said wire in said gripping position togrip said wire, wherein said wedges at their thicker ends are cut away on their inwardly facing surfaces, and wherein resilient means are provided embracing each said plurality of wedges adjacent their thicker ends, whereby, in the absence of a wire passing through said orifice and upon abutment of said sleeves against said gripper means, the tapered ends of said wedges are splayed out to facilitate insertion of a wire through said gripper means.

9. A pretensioning jack for pretensioning reinforcing wires comprising:

a. bearing means for bearing the jack against an anchoring plate, said anchoring plate having at least one orifice therethrough, a reinforcing wire passing d. traction means mounted on said body to draw said gripper-carrying head away from said bearing means to tension said wire,

e. a block slidablymounted in said bearing means to drive home said wedges to anchor said wire in said plate after tensioning of said wire, and

f. striker means to urge said block towards said plate to drive home said wedges.

10. A pretensioning jack according to claim 9 wherein:

the gripper-carrying head includes a gripper block provided with a longitudinal passage for each wire, the mouth of each passage furthest from the bearing means being conically shaped, and gripper means for seating in the conical mouths for gripping the reinforcing wires;

the actuating means comprises a release plate mounted to the end of the gripper-carrying head nearest the bearing means, the release plate having a longitudinal orifice for receiving each wire which is co-axial with the corresponding passage fomred in the gripper-carrying head, and a sleeve mounted in each orifice in the release plate in surrounding relation to the wire passing through the orifice, the sleeve being adapted tobe received by the corresponding passage in the gripper block;

the release plate is coupled to the gripper block for axial movement towards and away from the block;

the sleeve is further adapted upon movement of the release plate towards the block to enter the corresponding conical mouth and displace the gripper means therefrom; and

the jack further includes means for selectively moving the gripper block and release plate towards and away from each other.

11. A pretensioning jack according to claim 10 wherein the sleeves are of a length that their free ends always remain within the corresponding passages in the gripepr block when the release plate and gripper block are spaced apart.

12. In a pretensioning jack having a mouth for placement over reinforcing wires and having a grippercarrying head to grip the reinforcing wires prior to tensioning, the gripper-carrying head including a gripper block provided with a longitudinal passage for each wire, the mouth of each passage furthest from the mouth of the jack being conically shaped, and further including gripper means for seating in the conical mouths for gripping the reinforcing wires, a jack releasing apparatus comprising:

a release plate coupled to the side of the gripper block nearest the mouth of the jack and having a longitudinal orifice for receiving each wire which is co-axial with the corresponding passage formed in the gripper block, the release plate being coupled to the gripper block for limited axial movement towards and away from the gripper block;

a sleeve mounted in each orifice in the release plate in surrounding relation to the wire passing through the orifice, the sleeve being of a length and a diameter such that it is adapted to be received by the corresponding passage in the gripper block and is further adapted to enter the corresponding conical mouth and displace the gripper means therefrom when the release plate moves towards the gripper block; and

means for selectively moving the gripper block and plate towards and away from each other.

13. A jack releasing apparatus according to claim 12 wherein the free ends of the sleeves are located within the corresponding passages in the gripper block when the release plate and gripper block are spaced apart.

*zgx gy UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat N 3,762,686- Dated October 2, 1973 JUAN BAUTISTA RIPOLL GOMEZ Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 37, "axiial" should be axial--;

Col. 4, line 13, "sleves" should be -s1eeves-;

Col. 5, line 6, "form" should read from-'--;

Col. 6, line 19, "said gripper-carrying head etc." should be brought out to the left margin;

Col. 8, line 20, "fomred" should read --formed-.

Signed and sealed this 25th day er June 1971;.

(SEAL) Attest:

EDWARD M.F'LETGKER,JR. C. MARSHALL DANN Attesting Officer V Commissioner of Patents 

1. A pretensioning jack for pretensioning or prestressing reinforcing wires or rods and for anchoring the wires or rods after pretensioning or prestressing by wedging them with wedges sliding in corresponding seats of an anchoring plate comprising: a. an elongate hollow body for receiving the projecting portion of at least one reinforcing wire projecting from an anchoring plate for the same, said anchoring plate having i. an orifice for each said wire, ii. at least one wedge for each said wire, and iii. corresponding seating means for each said wire, b. bearing means at one end of said body for bearing against said plate and having a mouth to receive said projecting portion, c. gripper-carrying head within said body having gripper means for gripping said projecting portion, d. traction means at the other end of said body for drawing said gripper-carrying head longitudinally of said projecting portion away from said mouth to tension said wire, and e. striker means movable in said bearing means longitudinally of said projecting portion to force said wedge into engagement with its seating means to retain said wire in said plate after tensioning thereof, said gripper-carrying head including actuating means for releasing said projecting portion after tensioning of said wire.
 2. A jack according to claim 1, wherein said striker means comprises a block which is slidable in said bearing means longitudinally of said projecting portion, said block having an orifice for each said wire and a bearing face for bearing against said wedge, and wherein an annular piston slidable in a cylinder in said bearing means is provided for urging said block towards said anchoring plate to urge each said wedge into engagement with its seating means to retain said wire in said anchoring plate.
 3. A jack according to claim 1, wherein said traction means comprises a double-action hydraulic piston and cylinder arrangement, said piston having a tubular piston rod and a tie rod whiCh is axially slidable in said piston rod, wherein one end of said tie rod is provided with a retaining nut and wherein said gripper-carrying head is carried at the other end of said tie rod.
 4. A jack according to claim 1, wherein said gripper-carrying head comprises a gripper block and a release plate, each provided with an orifice for each said wire, the gripper block and release plate being spaced from one another along said projecting portion but are movable relative to one another in the longitudinal direction of said projecting portion, said gripper block being disposed further from said mouth than said release plate and having gripper means for each orifice and seating means therefor, said gripper means each being movable towards and away from said seating means between a gripping position and a non-gripping position, and resilient biasing means biasing said gripper means towards their respective gripping positions, said release plate carrying said actuating means, said actuating means comprising a sleeve lodged in each orifice of said release plate, which sleeves are disposed so that, upon movement of said gripper block and release plate towards each other, said sleeves abut against said gripper means and move them towards their non-gripping positions.
 5. A jack according to claim 4, wherein said traction means comprises a piston and cylinder arrangement, said piston being coupled with a tie rod, retaining means are provided at one end of said tie rod to prevent disengagement of said tie rod from said piston, wherein said tie rod is provided at its other end with screw thread means, and wherein one of said gripper block and release plate is freely rotatable on said tie rod and the other engages with said screw thread means, whereby, upon engagement of said grippper means with said projecting portion, rotation of said tie rod in one direction about its axis causes relative movement of the gripper block and release plate away from each other to cause said gripper means to move towards their gripping positions, rotation of the other one of the gripper block or release plate being prevented by said projecting portion, and whereby rotation of said tie rod in the other direction about its axis causes relative movement of the gripper block and release plate towards each other to cause said gripper means to move towards their non-gripping positions.
 6. A jack according to claim 5, wherein said release plate is freely rotatable on said tie rod, wherein a bearing member is provided on said tie rod so as to be rotatable thereabout but not axially displaceable thereon, wherein said gripper block is disposed between said bearing member and said release plate, and engages with said screw thread means, and wherein said resilient biasing means comprises springs which bear against said bearing member.
 7. A jack according to claim 4, wherein said traction means comprises a piston and cylinder arrangement, said piston being coupled with a tie rod, retaining means are provided at one end of said tie rod to prevent disengagement of said tie rod from said piston, wherein said tie rod is provided at its other end with a cage member having an internal screw thread, wherein said gripper block is screw threadedly engaged with said internal screw thread, wherein said release plate is freely mounted for free rotation on said cage about an axis co-axial with the axis of said tie rod, wherein a bearing member is provided fast with said first member and wherein said resilient biasing means comprises springs which bear against said bearing member.
 8. A jack according to claim 4, wherein said seating means each comprise a conical surface associated with each said orifice, wherein said gripper means comprise a plurality of wedges for each wire adapted to engage between said conical surface and said wire in said gripping position to grip said wire, wherein said wedges at their thicker ends are cut away on their inwardly facing surfaces, and wherein resilient means are provided eMbracing each said plurality of wedges adjacent their thicker ends, whereby, in the absence of a wire passing through said orifice and upon abutment of said sleeves against said gripper means, the tapered ends of said wedges are splayed out to facilitate insertion of a wire through said gripper means.
 9. A pretensioning jack for pretensioning reinforcing wires comprising: a. bearing means for bearing the jack against an anchoring plate, said anchoring plate having at least one orifice therethrough, a reinforcing wire passing through each said orifice and projecting therefrom, and wedges associatd with each said orifice to anchor said wire after tensioning thereof in said plate, b. a hollow body mounted on said bearing means to receive the projecting portion of said wire, c. a gripper-carrying head within said body having gripper means to grip said projecting portion, said gripper-carrying head including actuating means for releasing said projecting portion after tensioning of said wire, d. traction means mounted on said body to draw said gripper-carrying head away from said bearing means to tension said wire, e. a block slidably mounted in said bearing means to drive home said wedges to anchor said wire in said plate after tensioning of said wire, and f. striker means to urge said block towards said plate to drive home said wedges.
 10. A pretensioning jack according to claim 9 wherein: the gripper-carrying head includes a gripper block provided with a longitudinal passage for each wire, the mouth of each passage furthest from the bearing means being conically shaped, and gripper means for seating in the conical mouths for gripping the reinforcing wires; the actuating means comprises a release plate mounted to the end of the gripper-carrying head nearest the bearing means, the release plate having a longitudinal orifice for receiving each wire which is co-axial with the corresponding passage fomred in the gripper-carrying head, and a sleeve mounted in each orifice in the release plate in surrounding relation to the wire passing through the orifice, the sleeve being adapted to be received by the corresponding passage in the gripper block; the release plate is coupled to the gripper block for axial movement towards and away from the block; the sleeve is further adapted upon movement of the release plate towards the block to enter the corresponding conical mouth and displace the gripper means therefrom; and the jack further includes means for selectively moving the gripper block and release plate towards and away from each other.
 11. A pretensioning jack according to claim 10 wherein the sleeves are of a length that their free ends always remain within the corresponding passages in the gripepr block when the release plate and gripper block are spaced apart.
 12. In a pretensioning jack having a mouth for placement over reinforcing wires and having a gripper-carrying head to grip the reinforcing wires prior to tensioning, the gripper-carrying head including a gripper block provided with a longitudinal passage for each wire, the mouth of each passage furthest from the mouth of the jack being conically shaped, and further including gripper means for seating in the conical mouths for gripping the reinforcing wires, a jack releasing apparatus comprising: a release plate coupled to the side of the gripper block nearest the mouth of the jack and having a longitudinal orifice for receiving each wire which is co-axial with the corresponding passage formed in the gripper block, the release plate being coupled to the gripper block for limited axial movement towards and away from the gripper block; a sleeve mounted in each orifice in the release plate in surrounding relation to the wire passing through the orifice, the sleeve being of a length and a diameter such that it is adapted to be received by the corresponding passage in the gripper block and is further adapted to enter the corresponding conical mouth and disPlace the gripper means therefrom when the release plate moves towards the gripper block; and means for selectively moving the gripper block and plate towards and away from each other.
 13. A jack releasing apparatus according to claim 12 wherein the free ends of the sleeves are located within the corresponding passages in the gripper block when the release plate and gripper block are spaced apart. 